Process and apparatus for treating and utilizing vaporized gas in a ship for transporting liquified gas

ABSTRACT

The vaporized gas is first heated and then compressed. Thereafter, the gas is divided into two component flows with one component flow being sent to the ship&#39;&#39;s propulsion plant as a source of fuel. The second component flow is then further compressed and cooled in heat exchange relation with the continued flow of vaporized gas from the gas tanks. The cooled gas is then liquified and returned to the gas tanks.

United States Patent 11 1 1111 3,885,394 Witt et al. May 27, 1975PROCESS AND APPARATUS FOR 2,940,268 6/l960 Morrison 62/240 x TREATINGAND UTILIZING VAPORIZED 3,229,473 H1966 Schumacher et al. 62/240 X GASIN A FOR TRANSPORTING 3,303,660 2/1967 Berg 62/51 3,383,881 5/1968Bailey 62/50 x LIQUIFIED GAS 3,768,271 10/1973 Denis 02 55 Inventors:Karel Witt, Volketswil; Christian Trepp, Winterthur, both of SwitzerlandSulzer Brothers Ltd., Winterthur, Switzerland Filed: Dec. 10, 1973 Appl.NO.Z 423,122

Assignee:

Foreign Application Priority Data Dec. ll, 1972 Switzerland l7958/72U.S. Cl. 60/651; 60/671; 62/7; 62/54; 62/240 Int. Cl. Ftllk 25/08 Fieldof Search 114/74 A; 62/7, 50, 51, 62/54, 240; 60/651, 67]

References Cited UNITED STATES PATENTS Sattler et a]. 62/7 PROPULSIONUNIT Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. CaposselaAttorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin [57]ABSTRACT The vaporized gas is first heated and then compressed.Thereafter, the gas is divided into two component flows with onecomponent flow being sent to the ships propulsion plant as a source offuel. The second component flow is then further compressed and cooled inheat exchange relation with the continued flow of vaporized gas from thegas tanks. The cooled gas is then liquified and returned to the gastanks.

9 Claims, 1 Drawing Figure PROCESS AND APPARATUS FOR TREATING ANDUTILIZING VAPORIZED GAS IN A SHIP FOR TRANSPORTING LIQUIFIED GAS Thisinvention relates to a process and apparatus for treating and utilizingvaporized gas from a liquified gas tank of a ship for transportingliquified combustible gas.

As is known, the liquified gas transported in tankers and shipsvaporizcs or evaporates to some extent during travel due to the presenceof heat even in the best insulated vessels. In order to accept thisfact, proposals have been made to burn these gaseous products as fuel ina combustion device forming a part of the propulsion unit of the ship.

However, it has been found that the quantity of gas required to powerthe propulsion unit of a ship of normal size at normal speeds is lessthan the quantity of gas arising from vaporization.

Accordingly, it is an object of the invention to pro vide a proces andapparatus which can economically reduce vaporization losses byre-liquifying some of the gases.

It is another object of the invention to use the heat which is drawn offduring re-liquifaction to heat the gas vaporized in the liquified gastanks before this gas is compressed to a pressure suitable for thepropulsion unit.

It is another object of the invention to re-liquify some of the boil-offor vaporized gas from a liquified gas tank in order to reduce theeconomic loss due to vaporization of the liquified gas cargo.

Briefly, the invention provides a process in which a flow of vaporizedgas is drawn off at least one tank, compressed and divided into twocomponent flows. One component flow is then fed as a fuel to thepropulsion unit of a ship, for example, to a combustion device such as apiston engine, marine boiler or gas turbine plant. The other componentflow is further compressed and passed in heat exchange relation with thecontinued flow of vaporized gas from the tank to heat the flow ofvaporized gas prior to initial compression and to cool the gas componentflow. The cooled gas component flow is then reduced in pressure and fedback to the gas tank in at least partly liquified form. In this way, itis possible to use the low temperature of the full gas flow drawn offthe tanks to cool the second gas component flow over the entiretemperature range covered during cooling and, at least, partialliquifaction of this component flow.

The invention also provides an apparatus which includes means fordrawing off the flow of vaporized gas from a gas tank. a firstcompressor to compress the full flow and a first branch line to deliverone component flow to the propulsion unit. Also. a second compressor isused to receive and compress the second component flow and a heatexchanger is used to effect the heat exchange between the full flow andthe second component flow. A pressure reducing means is also connectedto the heat exchanger and the tank to deliver the at least partlyliquified gas back to the tank.

Due to the compression of the full flow in the first compressor. thesecond compressor does not need to operate at low suction temperaturesbut instead can operate at temperatures above C.

These and other objects and advantages of the invention will become moreapparent from the following detailed description and appended claimstaken in conjunction with the accompanying drawing in which:

The drawing illustrates a schematic view of a ship employing anapparatus according to the invention.

Referring to the drawing, a ship such as a liquified combustible gastanker has a propulsion unit (not shown) and a plurality of liquifiedgas tanks 10, only two of which are shown for simplicity. A means. suchas a line 11, is connected to each tank 10 to draw off a flow of gas.for example. methane or natural gas vaporized in each at a temperature,for example, of l50C and approximately at atmospheric pressure. The lineII is connected to a cooling passage 12 in a heat exchanger 13 todeliver the flow of vaporized gas to the passage 12 for heating, forexample. to 10C in a manner as described below. A line 14 extends fromthe exit end of the passage 12 to a compressor 15 to deliver the heatedflow for compression purposes. A delivery line 16 extends from thecompressor 15 through an aftercooler 17 and divides into two branchlines l8, 19. One line 18 conveys one component flow of the gas aftercooling in the aftercooler 17 to a combustion device (not shown) of thepropulsion unit (not shown), e.g. a combustion chamber of a steamgenerator. The line 18 is provided with a non-return valve 20 and apressure regulating valve 21 which maintains the pressure in the line 18at a predetermined value by means of a controller 21a.

The compressor 15 has a drive motor 22 whose speed is controlled by acontroller 23 according to the pressure in the line 11. The conditionsare such that when the pressure rises in the line 11 due to thevaporization of gas in the tanks 10 the speed is increased and viceversa.

The branch line 19 carries a second gas component flow into a secondcompressor 24 formed by a threestage piston machine driven by adiagrammatically indicated electric motor 25. Coolers 26 are alsoprovided between the various stages and where the gas component flowleaves the last stage.

A line 27 connects the outflow of the compressor 24 to the heatexchanger 13 so that the component flow of compressed gas comes intoheat exchange relation with the continued flow of vaporized gas from thetanks 10 and is cooled. A pressure reducing means including a line 28and a pressure reducing valve 29 e.g. an expansion valve is connected tothe heat exchanger 13 to receive the cooled component flow. The pressurereducing valve 29 is operated by a controller 30 so as to maintain thedesired pressure in the line 28. As shown, the valve 29 communicates viaa line 31 with a separator 33 which has a conventional level regulatingmeans 34 for operating a valve 35 in an exit line 36 to a pump 46. Thepump 46 serves to feed the liquified gas back to the tanks 10.

The separator 33 is connected by a line 41 to another cooling passage 42in the heat exchanger 13 so that volatile gas constituents, particularlynitrogen, can be drawn off along the line 41 and used to cool the gascomponent flow passing through the heat exchanger during liquification.These constituents then flow along a line 43 into the line 18, wherethey are added to the component intended for combustion.

As shown, the aftercooler 17 is connected by a line 38 to a source ofcoolant, for example, cooling water for cooling the gas flow through theline 16. Also, a regulating valve 380 is disposed in the line 38 and isoperated via a controller 39 in accordance with temperature variationsin the compressed gas in the line 16 so that a rise in temperature isfollowed by increased cool ing and vice versa.

The process of treating and utilizing the vaporized gas resides indrawing off the gas from the tanks via the line 11 into the heatexchanger 13. After heating, for example, to 10C, the gas flows alongthe line 14 into the compressor 15 and is compressed to a pressure ofapproximately 2 atmospheres absolute and heated to about 60C. Aftercooling to about 40C in the aftercooler 17, one component flow of gas isconveyed via the line 18 at the same pressure to the furnace (not shown)of a steam generator which produces steam for the turbines which propelthe ship.

The other gas component flow passes via the line 19 into the compressor24 and is compressed, for example, to about 42 atmospheres absolute andat a temperature of, for example, 40C. The gas then flows along the line27 into the zone of the heat exchanger containing the cooling passage 12and is cooled, for example, to 120C. The gas so cooled is dischargedfrom the heat exchanger 13 along the line 28 to the pressure reducingvalve 29, where the pressure is reduced to 2 atmospheres absolute, thatis, to the delivery pressure of the first compressor 15.

From the pressure reducing valve 29, the gas, now at least partlyliquified, flows along the line 31 into the separator 33 and collects inthe lower part of the separator 33. The re-liquified portion of the gasis then re turned to the tanks 10 via the line 36 by means of the pump46.

This invention is not restricted to the embodiment illustrated. On thecontrary, the gas component flow intended for combustion, instead ofgoing to a boiler furnace, can be fed to a diesel engine at a pressureof, for example, eight atmospheres absolute or to a gas turbine plantused to propel the ship. According to another alternative, the pressurereducing valve 29 is used to reduce the pressure to that in the tanks 10directly, omitting the separation of volatile gases.

Further, the invention provides an apparatus which is of simpleconstruction and which uses equipment on which no complicated demandsneed be made.

What is claimed is:

1. A process of treating and utilizing vaporized gas from a liquifiedgas tank of a ship for transporting liquified combustible gas comprisingthe steps of drawing off a flow of vaporized gas from at least one gastank;

compressing said flow of vaporized gas;

dividing the compressed flow of gas into at least two component flows;

feeding one of said gas component flows to a propulsion unit of theship;

compressing a second of said gas component flows;

passing said compressed second gas component flow into heat exchangerelation with a continued flow of said flow of vaporized gas to heatsaid continued flow of vaporized gas prior to compressing of saidcontinued flow of vaporized gas; and

thereafter reducing the pressure of said second gas component flow andfeeding a liquid part of the reduced pressure gas component flow to atleast one gas tank.

2. A process as set forth in claim 1 which further comprises the step ofcooling said flow of compressed gas prior to dividing into saidcomponent flows.

3. A process as set forth in claim 1 wherein the full flow of saidvaporized gas from one tank is compressed to a pressure demanded by thepropulsion unit.

4. A process as set forth in claim 1 which further comprises the step ofseparating volatile constituents from said reduced pressure gascomponent flow.

5. A process as set forth in claim 4 wherein said step of reducing thepressure of said second gas component flow includes expanding saidsecond gas component flow to a pressure corresponding to a deliverypressure of said vaporized gas from the tank and wherein the separatedvolatile constituents pass in heat exchange relation with said flow ofvaporized gas and are added to said one gas component flow.

6. An apparatus for treating and utilizing vaporized gas from aliquified gas tank of a ship for transporting liquified combustible gascomprising at least one gas tank;

means for drawing off a flow of vaporized gas from said tank;

a first compressor connected to said for drawing off means forcompressing the flow of vaporized gas;

a first branch line connected to said compressor to deliver onecomponent flow of the compressed gas flow to a propulsion unit of theship;

a second compressor connected to said first compressor to receive andcompress a second component flow'of the compressed gas flow from saidfirst compressor;

a heat exchanger connected with said means and to said second compressorto receive the flow of vaporized gas from said tank and the flow of thecompressed second gas component flow from said second compressor in heatexchange relation to heat the flow of vaporized gas and cool the secondgas component flow; and

a pressure reducing means connected to said heat exchanger and said tankto deliver the second gas component flow to said tank in at least partlyliquified form.

7. An apparatus as set forth in claim 6 which further comprises a secondbranch line between said first compressor and said second compressor toconvey the second gas component therebetween.

8. An apparatus as set forth in claim 6 which further comprises anafter-cooler downstream of said first compressor for cooling thecompressed flow of vaporized gas.

9. An apparatus as set forth in claim 6 which further comprises aseparator connected to said heat exchanger to receive the cooled secondgas component flow, a cooling passage in said heat exchanger, a linebetween said separator and said cooling passage for passing separatedvolatile gas constituents from the second gas component flow throughsaid cooling passsage, a line between said cooling passage and saidfirst branch line to deliver the volatile gas constituents thereto and aline between said separator and said gas tank to deliver liquified gasthereto.

1. A process of treating and utilizing vaporized gas from a liquifiedgas tank of a ship for transporting liquified combustible gas comprisingthe steps of drawing off a flow of vaporized gas from at least one gastank; compressing said flow of vaporized gas; dividing the compressedflow of gas into at least two component flows; feeding one of said gascomponent flows to a propulsion unit of the ship; compressing a secondof said gas component flows; passing said compressed second gascomponent flow into heat exchange relation with a continued flow of saidflow of vaporized gas to heat said continued flow of vaporized gas priorto compressing of said continued flow of vaporized gas; and thereafterreducing the pressure of said second gas component flow and feeding aliquid part of the reduced pressure gas component flow to at least onegas tank.
 2. A process as set forth in claim 1 which further comprisesthe step of cooling said flow of compressed gas prior to dividing intosaid component flows.
 3. A process as set forth in claim 1 wherein thefull flow of said vaporized gas from one tank is compressed to apressure demanded by the propulsion unit.
 4. A process as set forth inclaim 1 which further comprises the step of separating volatileconstituents from said reduced pressure gas component flow.
 5. A processas set forth in claim 4 wherein said step of reducing the pressure ofsaid second gas component flow includes expanding said second gascomponent flow to a pressure corresponding to a delivery pressure ofsaid vaporized gas from the tank and wherein the separated volatileconstituents pass in heat exchange relation with said flow of vaporizedgas and are added to said one gas component flow.
 6. An apparatus fortreating anD utilizing vaporized gas from a liquified gas tank of a shipfor transporting liquified combustible gas comprising at least one gastank; means for drawing off a flow of vaporized gas from said tank; afirst compressor connected to said for drawing off means for compressingthe flow of vaporized gas; a first branch line connected to saidcompressor to deliver one component flow of the compressed gas flow to apropulsion unit of the ship; a second compressor connected to said firstcompressor to receive and compress a second component flow of thecompressed gas flow from said first compressor; a heat exchangerconnected with said means and to said second compressor to receive theflow of vaporized gas from said tank and the flow of the compressedsecond gas component flow from said second compressor in heat exchangerelation to heat the flow of vaporized gas and cool the second gascomponent flow; and a pressure reducing means connected to said heatexchanger and said tank to deliver the second gas component flow to saidtank in at least partly liquified form.
 7. An apparatus as set forth inclaim 6 which further comprises a second branch line between said firstcompressor and said second compressor to convey the second gas componenttherebetween.
 8. An apparatus as set forth in claim 6 which furthercomprises an after-cooler downstream of said first compressor forcooling the compressed flow of vaporized gas.
 9. An apparatus as setforth in claim 6 which further comprises a separator connected to saidheat exchanger to receive the cooled second gas component flow, acooling passage in said heat exchanger, a line between said separatorand said cooling passage for passing separated volatile gas constituentsfrom the second gas component flow through said cooling passsage, a linebetween said cooling passage and said first branch line to deliver thevolatile gas constituents thereto and a line between said separator andsaid gas tank to deliver liquified gas thereto.